Near the beginning of the 20th century, it was recognized that a medically useful anatomical image could be obtained when a film containing a radiation-sensitive silver halide emulsion is exposed to X-radiation passing through a patient. Soon thereafter, it was recognized that a patient's exposure to X-radiation could be decreased considerably by placing an intensifying screen adjacent the film. An intensifying screen contains an inorganic phosphor that absorbs X-radiation and promptly emits light to expose the film.
Thus, a radiographic phosphor panel (or screen) contains a layer of phosphor that is a crystalline material that responds to X-radiation in an imagewise fashion. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. They are used to generate visible light immediately upon exposure of the panel to X-radiation. A radiographic film is positioned to intercept the generated visible light generated and is commonly pressed against the panel within a light-tight cassette.
U.S. Pat. No. 3,859,527 (Luckey) introduced a new concept of storage phosphor imaging. In this imaging system, a prompt emitting phosphor was replaced with a storage (“stimulatable”) phosphor that absorbs X-radiation and stores its energy until subsequently stimulated to emit light in an imagewise fashion as a function of the stored X-radiation pattern. Thus, the storage screen (now commonly referred to as an “image storage panel”) performs both functions of absorbing X-radiation like an intensifying screen and an image storage function like the radiographic film. This has allowed the radiographic film to be eliminated as a required element. Storage phosphors are generally different from prompt emission phosphors.
Image storage panels can be used in computed radiography wherein the panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation resulting in emission of radiation of a third wavelength. For example, a laser having a red light or infrared beam can be scanned over an image storage panel, resulting in a green or blue light emission. The emitted light is collected in appropriate apparatus and the resulting signal is processed electronically to provide a final image.
Various inorganic phosphors have been investigated in the last 30 years for use in image storage panels. Some very useful phosphors include alkaline earth metal fluorohalide phosphors, and particularly those containing iodide, as described in U.S. Pat. No. 5,507,976 (Bringley et al.), U.S. Pat. No. 5,523,558 (Bringley et al.), and U.S. Pat. No. 5,639,400 (Roberts et al.).
Degradation of final images in phosphor panels from environmental factors (such as humidity) and discoloration, and particularly yellowing, has been a concern for many years. For example, U.S. Pat. No. 5,789,021 (Dooms et al.) describes the use of various phenolic antioxidants to reduce yellowing of rubbery binders.
There has not, however, been agreement as to the source of discoloration in phosphor panels. The sources of the discoloration are not always apparent but in some instances, the presence of water that hydrolyzes the phosphors may be one of the causes. In addition, other investigators have evaluated the effects of free halogen and particular free iodide that may cause yellowing seems to be most prevalent when the phosphors contain iodide because of the likely formation of iodine. Various stabilizers have been proposed to solve this problem as noted in U.S. Pat. No. 5,523,558 (noted above) including the incorporation of phosphites, organotin compounds, epoxy compounds, and specific metal salts of organic acids (see U.S. Pat. No. 4,900,641 of Kohda et al.), oxysulfur reducing agents, thiosulfates, and metal oxides. U.S. Pat. No. 5,630,963 (Leblans et al.) describes the use of N-heterocyclic compounds as stabilizers in phosphor screens. U.S. Pat. No. 5,639,400 (noted above) describes the use of monocyclic compounds containing a quaternary nitrogen atom in storage panels to increase the transmitted stimulated luminescence lost from released iodine.
PROBLEM TO BE SOLVED
The progress toward the protection of image storage panels against discoloration by moisture or free halogen has greatly accelerated the practical use of these elements in computer radiography. Despite these advances, new problems have emerged even when the image storage panels are not exposed to high humidity. New and unusual artifacts have been observed in such panels that have no obvious defects during visual inspection. In particular, some image storage panels used in hospital radiology facilities have been unexpectedly found to display “minus density artifacts” after imaging. These defects typically appear in the computer-rendered images and may be either shown as straight lines or highly curved lines. No obvious defects such as scratches in the overcoat layer or phosphor layer were found and there was no visible discoloration of the panel.
Investigation of the problems noted above has revealed that a number of disinfectants used in clinical environments may cause imaging defects in phosphor panels. Specifically, phosphor panels that have been lightly wiped with such disinfectants may appear undamaged to the unaided eye, but are subsequently found to exhibit the “minus density artifacts” after imaging.
In addition, degradation may be accelerated by exposure to radiation (especially UV radiation). Storage panels are normally exposed to such radiation from fluorescent lamps used to “erase” images from storage panels.
There remains a need for additional protection for image storage panels against discoloration or image defects beyond the teachings of the art. In particular, there is a need to protect iodide-containing storage phosphor panels.